A fluid such as the atmosphere is a many-body system that has a vast amount of degrees of freedom in terms of statistical physics and thus obeys the second law of thermodynamics controlling the irreversible evolutionary direction of a many-body system. This study shows that incorporating the thermodynamic irreversibility law into a hydrodynamic system leads to a noticeable improvement of a numerical weather prediction model. This is demonstrated by improved simulations from a set of numerical experiments using a global spectral model which incorporates the second law.